I’m a lover of wildlife – especially birds, and venues in the “big-game” ballpark: my cruise would thus be to places good in themselves for that, or as setting-off points for locations inland (have been quite generous to self, re the latter). OP allows self Antarctica and the Galapagos as destinations in their own right among the 20, so I am following suit. Am in the UK, so it would seem to work well to leave from, and return to, a UK east coast port; have chosen Newcastle-upon-Tyne.
Newcastle
(1): Gdańsk – for the Białowieza Forest nature reserve
(2) : Constanţa (Romania) – for the Danube delta – wonderful bird-life
(3): Istanbul – for various destinations in Turkey (a birder’s paradise)
Via Suez Canal to
(4): Kolkata – Indian wildlife reserves, e.g. Kaziranga (N-E India), has tigers and rhinos
(5): Padang (Sumatra, Indonesia) – Kerinci Seblat national park huge in size, with a fantastic wealth of wildlife
(6): Darwin – tropical Australia – Kakadu National Park etc.
(7): Galapagos Islands
(8): Hobart – Tasmania has also been a total “must” for me
(9): Melbourne – “regular” Australia; and hoping for platypuses
(New Zealand, you have an enormous number of devotees already, so I’m sure you can live with my not being a very big fan, and – having only 20 “slots” worldwide – preferring Australia. NZ’s native wildlife has to a very large extent vanished under encroachment by introduced species.)
(10): Dar-es-Salaam --for the Serengeti, etc.
(11): Toamasina: chief port of Madagascar (island in a bad way environmentally, but the native fauna – such of it as survives – is unique and fascinating)
(12): Durban – Kruger National Park, and things South African in general
(13): Walvis Bay, coast of Namibia – desert-cum-swampland game reserves in Namibia and Botswana
(14): Pointe Noire (Congo-Brazzaville) or Libreville (Gabon) – a look at tropical Africa
(15): Antarctica
(16): Bahia Blanca or Buenos Aires, Argentina – “Pampas” and adjoining regions
(17): Manaus (800 miles up the Amazon, but ocean-going ships can get there) – Amazon forest
(18): Colón (Panama) or Limón (Costa Rica) – wonderful birdlife
(19): Portland, Maine – “northish-eastish” North America, generally
(20): Reykjavik, Iceland
Newcastle